Oxidation of organic compounds and manufacture of phthalic anhydride



Invenorn-v Sam B. B ker* Nov. 16, 1948. s. B. BECKER OXIDATION OFORGANIC COMPOUNDS AND MANUFACTURE OF PHTHALIG ANHYDRIDE Filed Feb. 1e,1945 .Se ESM mfoob Patented Nov. 16; 1948 OXIDATION OF ORGANIC COMPOUNDSAND MANUFACTURE OF PHTHALIC AN HYDRIDE Sam B. Becker, Chicago, Ill.,

0il Company, Chicago, Ill.,

Indiana assignor to Standard a corporation of Application February 16,1945, Serial No. 578,310 6 Claims. (Cl. 260-342) This invention relatesto oxidation of organic compounds, such for example as the manufactureof phthalic anhydride by controlled oxidation of naphthalene, and itpertains more particularly to an improved method and means for effectingsuch oxidation. This is a continuationin-part of my copendingapplication Serial 400,134 filed June 27, 1941, now United States PatentNo. 2,373,008.

An object of my invention is to provide an improved method and means forremoving heat developed by the oxidation of naphthalene hydrocarbons tophthalic anhydride and to obtain a closer temperature control in suchreactions than has heretofore been possible.

A further object of the invention is to provide an improved method andmeans for converting oil renery by-products containing large quantitiesof alkyl naphthalenes into phthalic anhydride.

A further object is to decrease the cost of manufacturing phthalicanhydride and to increase the yields of this compound obtainable fromcrude naphthalene or alkyl naphthalene charging stocks. Other objects ofthe invention will be apparent as the detailed description thereofproceeds.

In practicing my invention I effect the oxidation of the naphthalenehydrocarbons by means of a finely divided solid or powdered catalystwhich is maintained in turbulent dense phase suspension in the air whichpromotes the oxidation. A uniform temperature prevails throughout theentire zone occupied by turbulent dense phase suspended catalystparticles and hot spots and local overheating are entirely avoided. Heatmay be abstracted from the oxidation zone by means of suitable heatexchangers provided that the heat exchange surfaces do not interferewith the turbulent catalyst phenomena exhibited by dense phase suspendedcatalyst with critically controlled gas velocities. In order to insureagainst any interference with the turbulent catalyst phenomena I maymount the heat exchange surfaces substantially vertically around theperiphery of the oxidation zone or I may employ spaced vertical heatexchange surfaces within the oxidation zone itself when such surfacesare so spaced as to avoid interference with the dense phase phenomenaand adequate means are provided for insuring distribution of introduceduids.

As distinguished from the invention claimed in my copending application,the present invention utilizes spaced heat exchange surfaces in theoxidation zone itself and it utilizes the turbulence of the catalyst inthe oxidation zone itself to absorb the heat of oxidation and give upsaid absorbed heat to a heat exchange iiuid which is in indirect butheat exchange relationship with the contacting zone. 'I'he space abovethe dense catalyst phase is preferably maintained at a lower temperaturethan the dense catalyst phase itself and to effect still furtherquenching a relatively cool uid may be introduced into the gaseousproducts prior to or simultaneously with the separation of residualcatalyst particles therefrom.

The turbulent dense phase suspended catalyst phenomenon has been mostconclusively demonstrated in connection with powdered solids having aparticle-size of about 10 to 100 microns, i. e., particles of about 200to 400 mesh or liner. Such solids in settled or compacted state may havea bulk density of about =35 to 40 pounds per cubic foot. When subjectedto mild aeration with gas velocities of about 0.05 to .5 feet per secondthese solids behave as a liquid with a bulk density of about 25 or 30pounds per cubic foot. When the vertical velocity of the gas is about 1to 3 feet per second, particularly about 1%, to 21/2 feet per second thecatalyst becomes suspended in a turbulent dense phase or mass having abulk density of about 10 to 20, for example 15 pounds per cubic foot.Catalyst particles may be carried upwardly and beyond this dense phaseby the ascending gasesV into a dispersed catalyst phase which issuperimposed above the dense phase and separated therefrom by aninterface, the average density of the light or dispersed phase usuallybeing far less than 1 pound per cubic foot and usually only a .matter ofa few hundred grains per cubic foot. In the dense turbulent zone itselfit appears that the gases pass upwardly at a fairly uniform velocitywhile the suspended catalyst particles areconstantly moving in alldirections, cascading from top to bottom and being transported frombottom to top, so that there is a substantially uniform catalystdistribution throughout the entire zone. The necessary vertical gasvelocities for maintaining the dense turbulent phase conditionshereinabove described will depend to some extent on the density andcharacter of the catalyst particles as well as on catalyst particlesize; for extremely dense catalyst particles the necessary gasvelocities may be as high as 5 feet per second or more while with lessdense catalyst particles or particles with very rough catalyst surfacesthe necessary turbulence may be produced by gas velocities as low as .5feet per second. A feature of my invention in all cases is the use ofsuch vertical gas velocities as to maintain an interface and a verysmall catalyst density in the light dispersed phase above the interface,preferably below 500 grains per cubic foot. By maintaining the dilutephase cooler than the dense phase and substantially free from oxidationcatalyst undesirable side reactions are minimized.

The invention will be more clearly understood from the followingdetailed description read in conjunction with the accompanying drawingswhich form a part of this specification and in which similar parts aredesignated by like reference characters and in which:

Figure 1 is a schematic flow diagram of my process illustrating avertical section of an oxidation chamber equipped with peripheral heatexchange surfaces;

Figure 2 is a schematic flow diagram of my process illustrating avertical section of an oxidation chamber equipped with spaced verticalheat exchange surfaces throughout the oxidation zone, and

Figure 3 is a vertical cross-section of the reactor along the lines 3 3of Figure 2.

As a charging stock for my process I may employ hot pressed orcentrifuged naphthalene from coal tar, i. e., a grade of naphthalenehaving a melting point of about 77 to 79 C. (pure naphthalene melts at80 C.) A feature of my invention, however, is the utilization ofpetroleum refinery by-products and particularly the refractory stocksproduced by thermal or catalytic cracking or reforming. In thermal orcatalytic cracking processes for the production of gasoline from gasoils and heavier hydrocarbons one of the by-products is a refractorystock which may boil at about 400 to 550 F'. and which is characterizedby a large content of alkyl naphthalenes. The alkyl naphthalene contentof such refractory stock may be further concentrated by recycling to thecracking step or further cracking in a separate cracking step or byextraction with selective solvents for the removal of paraiiinichydrocarbons.

An important source of naphthalene and alkyl naphthalenes is therefractory stock produced in a process of catalytically converting a lowknock rating naphtha into high octane motor fuel by contacting thenaphtha vapors with a catalyst such as molybdenum oxide, chromium oxideor vanadium oxide supported on active alumina, the contacting beingeffected at temperatures of about 900 to 1000 F., pressures of about 50to 450 pounds per square inch, space velocities of about .2 to 2.0volumes of liquid feed per volume of catalyst space per hour, saidreaction being effected in the presence of hydrogen or recycle gascontaining hydrogen. This process is referred to as hydrocatalyticreforming or hydroforming'or dehydroaromatization. Theheavier-than-gasoline fraction which is produced in this reaction is arefractory stock sometimes referred to as reformate polymer and it mayhave an A. P. I. gravity of about 11, a distillation range of about 450to 600 F., a refractive index N2Tg) of 1.591

and a specific disperson of 264. 'I'his refractory stock contains largequantities of alkyl polycyclic aromatic hydrocarbons such as alkylnaphthalenes. The alkyl naphthalenes may be flu'ther tubes I2.Alternatively,

f introduced at the concentrated by solvent extraction or distillation,or both, for example, an 8 to 16% fraction obtained by distillation mayhave a distillation range of about 440 to 490 F., and A. P. I. gravityof about 17, a refractive'index of about 1.558 and a specific dispersionof about 224. A feature of my invention is the use of such by-productrefractory stocks produced in petroleum refining processes for theproduction of phthalic anhydride.

As catalysts for my process I may employ Vth or VIth group metal oxideseither unsupported or supported on suitable carriers such as alumina.silica gel, pumice, kieselguhr, or any other known catalyst supports.Activated alumina or silica gel may be impregnated with ammoniumvanadate or ammonium molybdate or both and then dried and heated toabout 900 to 1000" F. Silica hydrogel may be ball-milled with vanadiumoxide, molybdenum oxide or other catalytic oxides and the resultingdough dried and heated as in the previous example.

In the following example I will describe a system for employing acatalyst consisting of a mixture of vanadium and chromium oxidessupported on active alumina or silica gel but it should be understoodthat the invention is not limited to any particular catalyst compositionor preparation. If more active catalysts such as tin vanadatc areemployed, the oxidation temperatures should be lower than set forth inthe following examples because of the higher catalyst activity. By usingthe vanadium oxide or molybdenum oxide catalysts on relatively inertsupports the reaction may be more easily controlled, particularly whenthe catalyst itself acts as a heat absorber and heat carrier as will behereinafter described. I may use a finely divided or powdered catalysthaving a particle size of about 10 to 100 microns and containing about 2to 20% of vanadium oxide and molybdenum oxide. I will describe the useof such catalyst in a plant designed to produce about 2000 to 3000pounds per day of phthalic anhydride from naphthalene or by-productpetroleum refractory stocks. I

The simplest systemfor practicing my invention is illustrated in Figure1 wherein I provide cylindrical reactor I II about 2 or 3 feet indiameter and about 5 to 10 feets'ror more in height. The reaction isprovided with a cone-shaped bottom II: the sides of which are relativelysteep (i. e., about a 60 degree slope) so that the air which is base ofthe cone-shaped bottom will sweep any catalyst particles therefrom andprevent substantial catalyst deposition. About 250 to 500 pounds or moreof finely divided catalyst are placed in this reactor depending oncatalyst activity and on space velocities to be employed in the reactor.

Around the periphery o1' the reactor I employ substantially verticaltubes I2 which extend through the top and bottom reactor walls to upperheader I3 and lower header I4. Water may be introduced into the lowerheader through line I5 and steam withdrawn from the upper header throughline I6 suitable provisions being employed for regulating the pressureof the generated steam and regulating the water level in of course, Imay employ diphenyl, diphenyl oxide, a mixture of diphenyl and diphenyloxide (Dowtherm), mercury, or any other heat exchange fluid in tubes I2and I may generate steam by circulating said fluid through the tubes ofa boiler outside of the reactor itself.

In or above the top of the reactor I provide cyclone separators forremoving catalyst particles from the gases leaving the reactor. Thusgases from the top of the reactor may be introduced by inlet pipe I1 toprimary cyclone I8 which is provided with a dip leg I9 extending to thelower part of the reactor. Gases from the primary cyclone are introducedby line 20 -to secondary cyclone 2| which is provid-ed with dip leg 22.Gases from the secondary cyclone may be passed through oneor moreadditional internal cyclone stages or they may be withdrawn 'throughline 23 to external catalyst separation means 24 which may be additionalcyclone separators or may be an electrostatic precipitator or otherconventional separation means. Catalyst from this external separator isreturned to the system through line 25.

The catalyst-free reaction products are then passed through line 26 andpressure reduction valve 21 yto heat exchanger 28 which may be a wasteheat boiler for generating 15 pound process steam, the water beingintroduced into the eX- changer 'through line 29 and the steam beingwithdrawn through line 30. `The cooled gases and reaction yproducts arethen Iintroduced through line 3| into separati-on chamber 32 from whichgases are withdrawn through line 33 and the crude phthalic anhydride isperiodically or continuously removed by line 34 or by any otherconventional means. The specic method of fractionating the reactionproducts forms no part of the present invention and it will, therefore,not be described in further detail. Relatively pure phthali-c anhydridemay be separated from any unreacted naphthalene and from otherconversion products by conventional processes of fractional sublimation,distillation, crystallization, etc.

Settled catalyst in dip legs or pipes I9, 22 and 25 must be maintainedin uent condition to avoid plugging or bridging. The internal d'ip legsmay terminate above closure members 35 mounted on hollow stems 36extending through the bottom wall to external operating means 31 and agas such as steam or air may be introduced through line 38 anddischarged from the upper part of closure member 35 through suitablevents for dispersing catalyst 'into the reactor when the closure is inopen position, and for aerating or blowing out the dip legs when .theclosure members are in their upper closed posit-ion against the bottomof the dip legs. Similarly catalyst in pipe 25 may be aerated by air orsteam introduced by line 39 and may be discharged into pipe 4I inamounts regulated by valve t0.

Air is introduced through line 4I at a pressure of about l to 20 poundsper square inch and in amounts of about 500 to 3000, for example about2000 pounds per hour (about 25,000 cubic feet per hour measured atstandard conditions of temperature and pressure). This air picks upcatalyst from the base of pipe 25 and introduces it at the base oireactor IB wherein lit likewise suspends the catalyst returned to thebase of the reactor through dip legs I9 and 22.

Naphthalene vapors may be introduced with the air lin line 4I or may beintroduced into the reactor at various levels through pipes 42, 43 or44, the naphthalene charge being about 100 to 150, for example about 125pounds per hour. Prior to the introduction of naphthalene vapors thereactor may be brought to reaction temperature by burning a gaseous fueltherein, by passing hot iiue gases therethrough or by any otherconventionalmeans.

The reaction temperature will depend upon the specific catalyst and mayrange from about 500 to 1000 F. or more but with the vanadia-chromiaoxidation chamber I catalyst I prefer .to employ temperatures of about900 to 960 F., for example about 930 F. With known types of oxidationcatalyst however lower loxidation temperatures may be employed, forexample, in the approximate range of 500 to '100 F. The oxidation ofnaphthalene to phthalic anhydride liberates a considerable amount ofheat and the burning of alkyl side chains from alkyl naphth-alenesliberates even greater amounts of heat. Ii hot spots or localoverheating occurs in the reaction zone the oxidation will go too farwith the production of decreased yields of phthalic anhydride andincreased yields of carbon dioxide. An important feature of my inventionis the method of heat removal and temperature control for obtainingmaximum yields of valuable products.

Under the reaction conditions above lstated the vertical gas velocitylin the reactor will be about 1 to 3, for example, about 2 feet persecond. At such vertical gas velocities the catalyst will be maintainedin the turbulent dense phase suspended condition so that substantiallyidentical temperatures prevail throughout the entire reactor. The heatwhich is liberated in the reaction generates steam in pipes I2 (or ispicked up by a heat-absorbing iluid therein) and the -turbulent motionof the catalyst in the reactor carries the heat from the main body yofthe reaction zone to the heat exchange surfaces which surround thereactor. Remarkably close temperature control may thus be provided byregulating the pressure at which steam Iis generated in pipes I2 (orcontrolling the temperature and amount of heat exchange fluidcirculated) The contact time in the reactor may range from abou-t l to 4seconds depending upon the point in the reactor at which then-aphthalene vapors are introduced. With the particular catalystemployed a contact time of about 2 or 3 seconds should result inexcellent conversions. Catalyst is continuously removed from gases andvapors leaving the top of the reactor and this removed catalyst iscontinuously re-introduced into the dense turbulent catalyst suspension.The pressure on the reaction gases and vapors may be reduced to aboutatmospheric in valve 21 so that the recovery system operates at normalatmospherlc pressure.

It should be noted that after the gaseous,y

stream leaves the dense phase interface it enters a dilute phase whichis substantially free from catalyst so that oxidation is almostinstantaneously discontinued. I prefer to maintain this dilute phase ateven a lower temperature than th'e dense phase and this may beaccomplished by the use of the system described in Figure 2. In thesystem of Figure 2 heat exchange tubes I2a extend downwardly at spacedintervals from header 45 to distributor 42a, tubes I2a being closed attheir base and extending substantially to the distributor plate. Thedistributor 42a may .be a steel plate with spaced perforations about1/g-inch to l-inch or more in diameter and arranged to uniformly'distribute the incoming stream from line 4I throughout the space in theIl around tubes I2a. Internal tubes I2b extend downwardly from header 46to a point near but spaced from the closed bottom of tubes I2a. Coolingfluid, such as diphenyl, diphenyl oxide, a mixture of diphenyl anddiphenyl oxide (Dowtherm), mercury, or other heat exchange fluid, isintroduced through line I5 to the space I4a between plates 45 and 46wh'ich space is equivalent in function to header I4 of Figure 1. Thecooling fluid passes down wardly in the annular space between tubes I2aand |2b and then upwardly through tubes I2b to the space |3a betweenthe, plate 46 and the top of the reactor vessel, this last-named spacecorresponding to header I3 of Figure 1.

Except for the specific type and arrangement of heating surfaces andheaders it should be noted that the system of Figure 2 is substantiallythe same as that hereinabove described in connection with Figure 1. Inthe system of Figure 2, however, the heat exchange surfaces, aredistributed throughout the oxidation zone in such a manner as to oiTerno interference to the dense phase phenomena. The arrangement of thesesurfaces in this particular case is shown in Figure 3. With a 3-footdiameter reactor, pipes i2a may be about 4 inches in diameter and on 12-inch centers while internal pipes I2b may be about 21/2 to 3 inches indiameter. Generally speaking, pipes I2b should be about 2 inches to -6inches in diameter and they should be mounted in such a manner that theminimum distance between the outer surfaces of adjacent pipes is about 4inches. The minimum distance between pipes in the example is 8 inches.Preferably such minimum distance should be ,within the approximate rangeof 4 to 12 inches.

Figure 2 also illustrates the openings 42h in distributor 42a and itshows distributors 43a connected to line 43 for the introduction ofnaphthalene for oxidation. It should be understood however that noinvention is claimed in the specic type of distributors vemployed andthat any known type of distributors may be employed.

In th'e system illustrated in Figure 2 the dilute phase in the upperpart of the oxidation chamber is markedly cooled by the cooling fluidabove plate 45 and passing downwardly through the upper part of tubesI2a. This cooling prevents undesired side reactions particularly sincethe dilute phase is substantially free from catalyst ma-y terial. Thedilute phase may be of larger crosssectional area than the dense phaseby enlarging the upper part of the vessel as indicated at I a; thisprovides for improved settling and minimizes the amount of solids whichare removed from the dilute phase through' line 23. TheV amount ofsolids thus removed may likewise be minimized by employing a verticaldisengaging space above the dense phase level which is at least 2 feetand preferably upwards of 4 or 5 feet in height. the disengaging spacebeing the distance from the dense phase interface to the level at whichvapors are withdrawn through line 23.

Although the introduction of a quench fluid is usually not essential inmy system, such fluid may be introduced through line 41 into line 23 orline 25 or even into the dilute phase in the upper part Illa of theoxidation vessel. Cold air or other inert uid may be employed for thispurpose. Also relatively cold make-up catalyst material or catalystmaterial which has been Withdrawn from the oxidation chamber and cooledmay be introduced tp line 23 to effect quenching, such solidsintroduction tending to facilitate the handling of fines in recycle dipleg 26.

While the process has been described for the preparation of phthalicanhydride from naphthalene it should be understood that thetemperatures, catalysts, oxygen concentrations and operating conditionsmay be varied throughout a fairly wide range in order to effect anydesired extent of oxidation. Thus under proper conditions I may producesubstantial amounts of alpha-naphthoquinone which, in turn, may befurther oxidized to maleic anhydride. The phthalic anhydride may befurther oxidized to benzoic acid. Naphthols are difilcult to obtainbecause the presence of a hydroxyl group on the naphthalene ring greatlyincreases its activity toward oxygen, but under carefully controlledconditions even naphthols may be produced.

In the above description no special mention has been made of particularmethods for vaporizing the naphthalene charging stock but it should beunderstood that naphthalene may be vaporized in suitable coil heatersand introduced into the reactor at or below reaction temperature or itmay be vaporized by bubbling primary air through molten naphthalene andmixing secondary air with the vapors en route to the reactor. Byseparately vaporizing the naphthalene and introducing it at the properlevel in the reactor I may control the contacttime and thus determinethe extent of oxidation which is effected. A feature of my invention isthe much' larger yields of valuable oxidation products and lower yieldsof carbon dioxide than were obtainable in processes heretofore employed.

I have described the use of a powdered catalyst of about 200 to 400mesh, but it should be understood that larger particle sizes may beemployed if the vapor velocities in the reactor are properly modified tomaintain the desired turbulent dense phase suspension. In any event, itis important to maintain dense turbulent catalyst phase conditions inthe oxidation section of the contacting vessel superimposed by a dilutephase catalyst disengaging space and to avoid narrow catalystpassageways or obstructions in the oxidation zone which might interferewith the dense phase phenomena.

A particularly advantageous type of catalyst for my process is thespherical type prepared by the gelation of a colloidal solution whilesuspended as droplets in a liquid medium immiscible therewith. Forexample, an alumina sol prepared by the method described in U. S.Reissue Patent 22,196 may be distributed as droplets in oil or otherimmiscible liquid and coagulated and set to give spherical particles ofdesired size. Electrolytes may be employed for effecting the coagulationand setting but the use thereof may be dispensed with when thecomposition of the sol and the conditions of droplet suspension areproperly controlled. The spherical gel particles may be washed to removeundesirable ions, impregnated With salts of Vanadium, chromium or otherdesired catalyst material, dried and heated to a temperature in therange of 500 to 1000 F. more. Spherical catalysts may be prepared fromsilica sols in the same manner. These small spherical catalystparticles, sometimes called microspheres, seeds or beads are preferablyof small particle size, i. e. have an average diameter within theapproximate range of 3 to 300 microns, preferably within the aproximaterange of 10 to 100 microns. Their hard gel structure, rounded surfaces,and relative freedom from internal stresses and strains makes themhighly resistant to fracture and disintegration. The use of suchcatalyst particles reduces to a minimum the erosion of equipment, lines,and valves. Also the use of such catalysts reduces to a minimum thetendency toward catalyst carry-over from the oxidationr zone. Simplecyclone separation is adequate to prevent catalyst losses and evencyclone separators are not always necessary. The

. such bayonet type tubes may extend upwardly from the bottom of theoxidation zone, the header plates being mounted in the bottom instead ofin the top of the reaction vessel, suitable provision being made for theintroduction and distribution of introduced air above the headers andthe bayonet tubes extending upwardly into the dense catalyst phase atsuch spaced intervals as to prevent interference with the dense phasephenomena.

While the use of cyclone separators has been j described for preparingcatalyst from the light dispersed catalyst phase it should be understoodthat lters or any other separation means may be employed instead of orin addition to the cyclone separators. Also the cyclone separators maysuspended dense turbulent phase superimposed by a light dispersedcatalyst phase above the dense phase level, maintaining said denseturbulent catalyst phase at a substantially constant oxidationtemperature by continuously and in situ extracting heat from said densephase above the point at which air is introduced thereto, employing alarge vertical disengaging space above the dense phase level which is ata lower temperature lthan the temperature of the dense turbulent phase,withdrawing a gasiform stream from the upper part of the disengagingspace, separating catalyst particles from said last named stream,returning said separated particles to said dense phase and recoveringphthalic anhydride from said withdrawn stream after catalyst particleshave been separated therefrom.

3. The method of converting naphthalene hydrocarbons into phthalicanhydride which combe employed either internally or externally of theoxidation zone.

While I have described in detail certainpreferred embodiments of myinvention as applied to a particular process it should be understoodthat my invention is not limited to the specific systems or to thespecic operating conditions hereinabove set forth. Numerousmodifications and alternative procedures and conditions may be apparentto those skilled in the art from the above description.

I claim: s

1. The method of converting naphthalene hydrocarbons into phthalicanhydride which comprises maintaining in an oxidation zone a large massof solid oxidation catalyst particles having particle size chieflywithin the range of about 10 to 100 microns, passing a stream consistingessentially of air and naphthalene hydrocarbon vapors upwardly throughsaid mass of catalyst particles at a velocity in the range of about .5feet per second to about 5 feet per second and sufficiently low tomaintain the catalyst as a suspended dense turbulent phase superimposedby a light dispersed catalyst phase above the dense phase level,maintaining said dense turbulent catalyst phase at a substantiallyconstant oxidation temperature by continuously and in situ extractingheat from the dense phase above the point at which air is introducedthereinto, employing a vertical disengaging space above the dense phaselevel which is at least 2 feet in height, maintaining the dispersedcatalyst phase in the disengaging space at a lower temperature than thetemperature of the dense turbulent phase, separating catalyst particlesfrom the gasiform stream withdrawn from the upper part of thedisengaging space and returning said separated particles to said densephase.

2. The method of converting naphthalene hydrocarbons into phthalicanhydride which comprises maintaining in an oxidation zone a large massof solid, substantially spherical oxidation catalyst particles having anaverage particle size within the range of about 10 to 100 microns,passing a stream comprising oxygen and naphthalene hydrocarbon vaporsupwardly through said mass of catalyst particles at a velocity in therange of about .5 feet per second to about 5 feet per second andsufficiently low to maintain the catalyst as a prises maintaining in anoxidation zone a large fluidized mass of solid substantially sphericaloxidation catalyst particles having a particle size within the range ofabout 3 to 300 microns, passing a stream comprising air andnaphthalenediydrocarbon vapors upwardly through said mass of catalystparticles at a Velocity in the range of about .5 feet per second toabout 5 feet per second and sufficiently low to maintain the catalyst asa suspended dense turbulent phase superimposed by a light dispersedcatalyst phase above the dense phase'level, maintaining said denseturbulent catalyst phase at a substantially constant oxidationtemperature in therange of about 500 F. to 100 F. by continuously insitu extracting heat from said dense phase above the point at which airis introduced thereto, separating catalyst particles from product gasesbefore said product gases are withdrawn from theoxdation zone, andrecovering phthalic anhydride from withdrawn gases from which catalystparticles have been removed.

4. In the process of converting naphthalene hydrocarbons into phthalicanhydride by air oxidation in the presence of a, catalyst comprising anoxide of a metal selected from groups V and VI of the periodic system,the improved method of operation which comprises employing in theoxidation zone a deep mass of catalyst particles having a particle sizechiefly in the range of about l0 to 100 microns, distributing introducedair at the base of said zone, introducing naphthalene stream velocity inthe oxidation zone in the range of .5 to 5 feet per second andsufficiently low to maintain the catalyst as a suspended dense turbulentphase superimposed by a light dispersed catalyst phase in a catalystdisengaging space 5 which extends upwardly a substantial distance abovethe dense phase level, maintaining the oxidation zone at a substantialconstant temperature in'the range of 500 to 1000 F. by removing heat insitu directly from the dense phase above the point at which air isintroduced into said zone, separating catalyst particles from thegasiform stream while it is being withdrawn from the upper part of thedisengaging space, returning said separated particles to said densephase and recovering phthalic anhydride from the withdrawn stream aftercatalyst particles have been separated therefrom.

5. The process of claim 4 in which the naphthalene hydrocarbons arealkyl naphthalenes produced by the hydroforming of a. petroleum naphthaand which boil chieiiy within the range of 450 to 600 F.. which have arefractive index within the range of about 1.5 to 1.6 and which have anA. P. I. gravity within the range of about 10 to 20.

6. The method of converting naphthalene hydrocarbons into phthalicanhydride which comprises introducing said hydrocarbons at at least onelow point into an oxidation zone containing a deep mass of solidoxidation vcatalyst particles comprising vanadium oxide and having aparticle size chiefly within the range of 10 to 100 microns, introducingan air stream at the base of said zone at such a rate as to maintaintherein an upward gasiform stream velocity in the range of .5 to 5 feetper second and suiiiciently low to maintain the catalyst as a suspendeddense turbulent phase superimposed by a light dispersed catalyst phasein a catalyst disengaging space which extends upwardly a substantialdistance above the dense phase level, operating said oxidation zoneunder pressure of about 15 to 20 pounds per square inch at a temperatureof about 700 F., introducing about 5 to 30 pounds of air per pound ofnaphthalene hydrocarbons introduced, maintaining a substantiallyconstant temperature in the oxidation zone by removing heat in situdirectly from the dense phase above the point at which air is introducedinto said zone, separating catalyst particles from the gasiform streamwithdrawn from the upper part of the disengaging space, returning saidseparated particles to said dense phase l0 and recovering phthalicanhydride from said Withdrawn stream after catalyst particles have beenseparated therefrom.

SAM B. BECKER` 15 REFERENCES CITED The following references are ofrecord in the file of this patent:

UNITED STATES PATENTS 20 Number Name Date 1,916,473 Forrest et al July4, 1933 1,972,937 Jaeger Sept. 11, 1934 2,373,008 Becker Apr. 3, 19452,409,780 Mekler Oct. 22, 1946

